Class D amplifiers operate by generating a variable duty cycle square wave of which the low-frequency portion is essentially the wanted output signal, and of which the high-frequency portion serves no purpose other than to make the wave-form binary so it can be amplified by switching power devices in an output stage of the amplifier. A passive low-pass filter removes the unwanted high-frequency components and recovers the desired low-frequency signal. To maintain high efficiency, the filter is typically made with reactive components which store the excess energy until it is needed instead of converting some of it into heat.
Theoretical power efficiency of class D amplifiers is 100%. That is to say, all of the power supplied to it is delivered to the load and none is turned to heat. This is because an ideal switch in its on state will conduct all the current but has no voltage across it, and hence no heat would be dissipated. And when it is off, it will have the full supply voltage across it, but no current flows through it. Again, no heat would be dissipated. Real-world power MOSFET's are not ideal switches however and practical efficiencies well over 90% are common. Timing between switches must also be controlled such that both switches are not turned on at the same time which could damage the output stage. This time where one switch is turned off before the other is turned on is referred to as dead time.
One problem with operation of Class D amplifiers relates to the effects of the direction of load current on the PWM output signal of the amplifier. Thus, one direction of output current (e.g., high to low) can cause differences in the PWM output signal timing that are different from that when the output current changes in the opposite direction (e.g., low to high). Such differences in timing leads to a distortion in the output signal quality of the amplifier and is referred to as total harmonic distortion (THD).